Lapping is the process wherein fine abrasive particles suspended in a carrier substance are utilized to abrade the surface of a workpiece.
In the gear finishing process of lapping, an abrasive mixture, referred to as lapping compound, is introduced to the meshing tooth surfaces of two gears under load conditions. The application of the lapping compound is usually through nozzles near the meshing point of the teeth with the flow rate, pressure, temperature and consistency of the compound being significant factors in the effectiveness of the delivery system.
One such delivery system is shown in U.S. Pat. No. 2,541,283 to Praeg wherein delivery of the lapping compound is provided by a suspended tank containing a plunger to force compound through a tube to a nozzle adjacent the gears to be lapped.
Another method of delivering lapping compound to the lapping chamber of a gear finishing machine is disclosed by U.S. Pat. No. 1,796,484 to Slade. Lapping compound is pumped from a reservoir via a flexible hose to a nozzle. The lapping chamber includes a drain connected to a pipe to return the lapping compound to the reservoir after it has been applied to the gears being lapped.
U.S. Pat. No. 2,691,250 to McMullen et al. teach a lapping compound mixing and delivery system comprising a mixing tank having two paddle wheels. The tank is connected, via a trap, to a pump for providing lapping compound to the lapping chamber. The trap is intended to prevent abrasive material that settles out of suspension during times when the machine is not operating, from entering the pump.
The matter of particles settling out of suspension during times of no machine operation is discussed in U.S. Pat. No. 4,513,894 to Doyle et al. A supply system for delivering abrasive slurry to a polishing or grinding machine for metallurgical specimens includes a valve to direct the abrasive slurry back into the holding tank to assist in mixing the abrasive slurry when the machine is not operating. However, injecting compound at the top of the tank produces little mixing action. Furthermore, Doyle et al. disclose a sharp-cornered, flat bottom tank. In tanks with corners and flat bottoms, particles have a tendency to settle in the corners at the tank bottom and Doyle et al. teach no significant means to effect a complete mixing action to keep all abrasive particles in suspension in the tank. No manner of providing heat to the system is mentioned.
The primary factor in the successful application of lapping compound is to effectively control the consistency of the mixture. This involves temperature control (near 100 degrees Fahrenheit) to affect viscosity which in turn impacts the ability of the compound to flow through the dispensing medium, and mixing the compound to keep the abrasives in suspension with the oil, countering the tendency of the abrasives to settle out of the medium and rendering the compound as delivered to the application as weakened and, therefore, ineffective.
In most pumping systems, two types of pumps are generally utilized, electrical-driven centrifugal open impeller type or pneumatic type. Either type has been prone to failure due to conditions relating to the compound or its application.
The open impeller centrifugal type has experienced a short service life due to abrasion of the impeller. The open impeller design produces flow by forcing the fluid between the impeller vanes and the pump housing. Any influence of the compound to abrade and erode these surfaces causes increased clearances, deteriorates the impelling surfaces, and directly decreases the capacity of the pump.
Pressure of the fluid in the discharge port of the pump, caused by restrictions and valves in the delivery system, further accelerate the wear in there areas due to increased internal slippage. This phenomenon causes a premature dulling of the compound abrasives themselves, limiting their usefulness as a lapping medium, and creating heat which must be dissipated to the extent the compound temperature exceeds the desired value. In effect, pumping of the compound with this pump design causes an immediate and progressive decay in the output volume until the pump becomes ineffective for application.
Additionally, the entire pump must be replaced to effect repair since the pump housing is generally worn. This failure of the pump can occur in as little as 1000 production hours.
The diaphragm pump causes flow by alternating pneumatic force to diaphragm cavities. Through the use of valves, the progressive movement of these diaphragms displaces the fluid into the discharge port and to the application. The pump only displaces the fluid which will flow through the application and therefore does not generate heat through internal slippage as is typical with the centrifugal type. Additionally, the control of the flow rate can be easily regulated by adjusting the pneumatic force applied to the diaphragms.
Diaphragm pumps are prone to frequent failure due to rupture of the diaphragm because of fatigue, failure of the valves regulating flow to the diaphragm due to abrasion, and, failure of the pneumatic valves sequencing the diaphragm actuations as a result of moisture and rust in the supplied air.
A major source of failure in any lapping compound supply system is the presence of metal chips from the workpieces which find their way into the lapping compound. In some instances, workpieces, especially gears, are not chamfered to remove burrs after cutting. When such workpieces are heat treated, burrs that are present become very hard and sharp. These burrs are removed from the workpiece by the lapping process and subsequently make their way through the compound supply system causing damage to the pump, especially to diaphragms which are easily punctured, clogging the valves and piping system, or, ruining other workpieces by being reintroduced into the lapping chamber along with the lapping compound.
It is an object of the present invention to eliminate the need for separate mixing and pumping devices while improving the temperature and dispersion homogenization of the lapping compound.
It is another object of the present invention to increase pump service life, improve temperature control of the compound, and allow easier access to the reservoir for cleaning.
It is a further object of the present invention to eliminate any requirement for pneumatic service to the pumping system.